Heterojunction bipolar transistor and amplifier including the same

Abstract

An N-type collector layer is partially formed on an N + -type collector contact layer. The N-type collector layer includes a second N-type collector layer that is partially formed on the N + -type collector contact layer and relatively hard to deplete, and a first N-type collector layer that is formed on the whole surface of the second N-type collector layer and depleted relatively easily. A P + -type base layer including a high concentration P-type impurity is formed on the whole surface of the first N-type collector layer.

1 . A heterojunction bipolar transistor comprising a first conductivity type collector layer, a first conductivity type emitter layer, and a second conductivity type base layer interposed between said first conductivity type collector layer and said first conductivity type emitter layer, 
 said first conductivity type collector layer including:    a first conductivity type first collector layer making contact with said second conductivity type base layer; and    a first conductivity type second collector layer making contact with said first conductivity type first collector layer, wherein    said first conductivity type first collector layer is entirely depleted upon being supplied with a lowest collector voltage, said lowest collector voltage being the lowest conceivable collector voltage to be used, and    said first conductivity type second collector layer has a carrier concentration higher than space charge concentration based on a normal collector voltage, and a depletion layer formed therein upon being supplied with said normal collector voltage, the ratio of the thickness of said depletion layer being not more than  20 % with reference to the thickness of a depletion layer formed in the whole of said first conductivity type collector layer.    
   
   
       2 . The heterojunction bipolar transistor according to  claim 1 , wherein said first conductivity type second collector layer has a carrier concentration lower than space charge concentration based on a highest collector voltage, said highest collector voltage being the highest conceivable collector voltage to be used, and is entirely depleted upon being supplied with said highest collector voltage.  
   
   
       3 . The heterojunction bipolar transistor according to  claim 1 , further comprising: 
 a first conductivity type high carrier concentration collector contact layer making contact with said first conductivity type second collector layer; and    a first conductivity type electric field relaxation layer interposed between said fist conductivity type second collector layer and said first conductivity type high carrier concentration collector contact layer, and having an impurity concentration higher than said first conductivity type second collector layer and lower than said first conductivity type high carrier concentration collector contact layer.    
   
   
       4 . The heterojunction bipolar transistor according to  claim 2 , further comprising: 
 a first conductivity type high carrier concentration collector contact layer making contact with said first conductivity type second collector layer; and    a first conductivity type electric field relaxation layer interposed between said fist conductivity type second collector layer and said first conductivity type high carrier concentration collector contact layer, and having an impurity concentration higher than said first conductivity type second collector layer and lower than said first conductivity type high carrier concentration collector contact layer.    
   
   
       5 . An amplifier comprising the heterojunction bipolar transistor recited in  claim 1  as an amplifying element.  
   
   
       6 . An amplifier comprising the heterojunction bipolar transistor recited in  claim 2  as an amplifying element.  
   
   
       7 . An amplifier comprising the heterojunction bipolar transistor recited in  claim 3  as an amplifying element.  
   
   
       8 . An amplifier comprising the heterojunction bipolar transistor recited in  claim 4  as an amplifying element.